*{
    padding: 0;
    margin: 0;
    font-size: 14px;
    font-family: Arial, Helvetica, sans-serif;
    color: #242424;
    -webkit-user-select: none;
}

ul,li{
    list-style: none;
}

#box{
    position: relative;
    width: 850px;
    height: 400px;
    margin: 0 auto;
    
    >li{
        width: 200px;
        height: 200px;
        float: left;
        border: 1px solid green;
        
        img{
            width: 200px;
            height: 200px; 
        }
    }
}

.mask{
    position: absolute;
    width: 400px;
    height: 400px;
    left: 0;
    top: 0;
    display: none;
    >img{
        width: 100%;
        height: 100%;
        
    }
}